The application generally relates to a solid state relay protective device. The application relates more specifically to a solid state relay with a feedback circuit for interrupting an input control signal.
In contrast to mechanical relays that use movable contacts to open or close a circuit, a solid state relay (SSR) is a semiconductor switching device such as a silicon controlled rectifier (SCR), a metal-oxide semiconductor field effect transistor (MOSFET) or similar transistor device. A solid-state relay controls the flow of current. A low-level driving signal can control a larger flow of current through the solid-state relay, similar to the way that a mechanical relay may use a low power signal to switch a large current. In contrast with mechanical relays, solid-state relays provide faster switching times, no contact bounce or acoustic noise, the ability to synchronize the switching action with an external event such as the zero crossing of an AC waveform, and no wear mechanisms such as arcing or pitting of contacts.
A solid-state relay may fail, in response to current surges that result, for example, from switching an inductive load, fault current caused by a short circuit, or excess temperature. Excess current causes the solid-state relay to overheat, which may destroy the semiconducting junction that produces the switching action. The switching element of a solid-state relay has a much lower thermal inertia than a mechanical relay, and the safe operating temperature of the solid-state relay switching element is much lower than that of a mechanical relay. These characteristics make the solid-state relay much more sensitive than a conventional mechanical relay to transient overloads such as inrush current due to a motor startup.
Further, solid-state relays often fail in the closed mode, which can leave a circuit energized with no means of removing the load current.
It would be desirable to provide a protective circuit for a solid-state relay with the ability to remove a load current in the event that the solid-state relay fails. Other features and advantages will be apparent from the following disclosure.